Temperatures for Growing Soybeans

Soil Temperature

• Like most beans, soybeans respond best to warm soil. They won't germinate well until soil temperature reaches 65 F, and the best and quickest germination is at 85. An absolute minimum of 55 to 60 F is needed. These temperatures may occur in May or later, depending on your location. Fast germination is essential for healthy plants; plants are susceptible to diseases if their seeds sit in wet soil too long. Once seedlings have emerged, soil temperatures between 60 and 65 F are necessary for continued growth.

Air Temperature

• Air temperature is a reliable sign of planting date for soybeans. Ideal air temperature is about 63 to 77 F for the formation of flowers and pods. Soil should be fertile and well-drained. Protect the plants from cold or freezing weather, which will slow growth substantially. Soybeans are not cold-tolerant, and air temperatures below 28 F can kill them. Sustained frost can diminish yield by 80 percent, according to the Ontario Ministry of Agriculture, Food, and Rural Affairs.

Planting Date

• Research shows that the planting date can greatly affect soybean yield. Planting early in the year is liable to produce larger crop yields but can be risky due to the chance of late frost. If soybeans are planted too early when soil is cold and wet, significant delays in germination could occur. Researchers at the University of Wisconsin found that early May planting produced the best yield under ideal growing conditions. Planting dates of June 1 or later performed well with an early-maturing variety, but planting after June 15 proved unsuccessful.

Other Growth Conditions

• Soybeans grow best in soil with a pH range between 6.5 and 7.5. Soil should be fertile and well-drained. Provide full, direct sun and allow plenty of airflow so foliage remains dry. Like all legumes, soybeans take part in nitrogen fixation, meaning they derive nitrogen from bacteria. This process does not begin right away, so it is helpful to add a light fertilizer when plants are young and if soil is not rich enough. Yellowing or pale leaves are often a sign of nitrogen deficiency.